Passenger vehicles with no tailpipe emissions such as electric vehicles are growing in popularity. Nevertheless, the vast majority of vehicles on the road are still powered by engines that combust fossil fuels such as gasoline and diesel. These vehicles contribute to significant air pollution both in their immediate vicinity on the roadway and in the air of all major cities around the world. Air pollution also comes from other human activities such as factories and heavy industries, as well as from natural phenomena such as plants, animals, volcanoes and forest fires.
In many areas, commuting by vehicle is a daily routine for a large part of the population. For example, it is not uncommon to spend a total of an hour or more riding in a car each day, sometimes several hours, to get to and from work. During this time, the driver and passengers are breathing air drawn from the immediate outside of the vehicle. Because of the problems mentioned above, this air can contain significant amounts of pollutants in form of particles and gasses that the occupants inhale into their lungs while riding in the car. Despite the use of traditional air filters in vehicles' heating, ventilation and air conditioning (HVAC) systems, the air delivered into the passenger compartments of today's vehicles contains significant amounts of pollutants. This can negatively affect human health.